This invention relates generally to cellular radio telecommunications and more particularly to situating antennas to obtain cellular antenna arrays.
It is well known by those skilled in the art that for a radio telecommunications system to be efficient it must make optimum reuse of allocated frequencies to minimize the radio frequency spectrum used in a particular geographic area. Frequency reuse is particularly important in crowded metropolitan areas where frequency allocation may be difficult to obtain. However, the optimization of frequency reuse is bounded by the amount of co-channel interference that may be deemed acceptable in a telecommunications system.
Early techniques used the so called large zone approach wherein high powered transmitters were used to cover large sections (cells) of geographic area at a time. While equipment efficient, the large zone approach made poor use of the frequency spectrum. As the useable spectrum became more crowded and frequencies harder to obtain, the large zone system gave way to alternative cellular concepts.
An advance in the art was the progression to the small zone system. The small zone system employed low powered transmitters to cover small sections of geographic area at the expense of equipment. The small zone system requires a plurality of satellite transmitters and antennas located in each cell configured in various arrangements to illuminate the cell. One example of a small zone illumination technique could be the combination of an omni-directional antenna, together with 60.degree. sectoral horn antennas positioned in the center of the cell illuminating radially outward. Thus, this implementation requires seven antennas per cell. An alternative example, offering a cost improvement, could be the use of directional antennas located on the perimeter of the cell placed 120 .degree. apart radiating inward thereby covering 360.degree. of the cell. Thus, this improved technique required only three antennas per cell.
The small zone system provides more efficient use of the frequency spectrum by placing the lower powered transmitters (cells) closer together in order to reuse the same frequencies in the same metropolitan area. However, this frequency reuse technique has its limitations. In particular, optimization of frequency reuse is bounded by the amount of co-channel interference that may be deemed acceptable in a telecommunications system. If, for example, the small zones are placed close together, a subscriber of a telecommunications system would experience co-channel interference anifesting itself in the form of cross-talk over his telecommunications channel.
Therefore, the need in the cellular telecommunications marketplace is to develop a system to balance the conflicting needs of high frequency reuse and low co-channel interference.